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Liu B.-R.,CAS Institute of Geology and Geophysics | Liu B.-R.,University of Chinese Academy of Sciences | Song H.-B.,Tongji University | Guan Y.-X.,Guangzhou Marine Geology Survey | And 5 more authors.
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2015

Gas Hydrate reservoir in the northeastern continental slope of South China Sea(SCS) have been confirmed by investigations and well drillings. Also, large amounts of methane-derived authigenic carbonates (MADC) were discovered in 2004 among this area. Based on sub-bottom profiles and seismic profiles, attributes and features associated with cold seepage system in the northeastern continental slope of SCS were analyzed. A seismic line cross the northeastern continental slope of SCS was used to show local sedimentary environment. Sub-bottom data were pieced together and denoised. Shallow gas, fluid migration and morphology features associated with cold seepage in processed sub-bottom profile were identified and discussed, with assistance of corresponding seismic lines and bathymetric charts. From observation of sub-bottom profiles and comparison with seismic and bathymetry data, cold seepage related features were listed as follow: (1) Acoustic plume (suspected of being cold seep) was found in a sub-bottom profile, with height of about 30 meters and width of 50 meters. (2) Mud volcanoes were found in this area, and caused discontinuity of Bottom Simulating Reflectors in seismic profiles. (3) Acoustic voids, the most frequent features in this region, had two types: “narrow” acoustic void and “broad” acoustic void. “narrow” acoustic void had width of 80~400 m and no layers information, “broad” acoustic void had width over 1000m and weak layers information. (4) The area of “narrow” acoustic void overlaid with mud volcanoes concentrated area, and the acoustic plume was located around “broad” acoustic voids. Cold seepage activities exist on the northeastern continental slope of SCS, both in history and in present. The results suggest that “narrow” acoustic voids on sub-bottom profiler correspond to fluid migration path, while “broad” acoustic voids possibly related with shallow gas accumulation along rock layers. The relationship between cold seepage system and gas hydrate in the northeastern continental slope of SCS worth further investigation. ©, 2015, Science Press. All right reserved. Source

Sun Y.,CAS Qingdao Institute of Oceanology | Wu S.,CAS Qingdao Institute of Oceanology | Dong D.,CAS Qingdao Institute of Oceanology | Ludmann T.,University of Hamburg | Gong Y.,Guangzhou Marine Geology Survey
Marine Geology | Year: 2012

Drilling for gas hydrates at the northern continental margin of the South China Sea (SCS) provides a unique insight into the formation of the gas hydrate system in fine-grained sediments, even though most high concentrations of hydrates are found in coarse sediments. Detailed studies of 3D seismic data from a deepwater basin (Baiyun Depression) of this region reveal the presence of overpressured fluid flow, mostly manifested in gas chimneys. Gas chimneys are commonly characterized by disrupted reflections (DR), dim amplitude anomalies (DA), and enhanced reflections (ER) on conventional seismic profiles. Gas chimneys are also characterized by chaotic, low-continuity and low-frequency on instantaneous frequency and instantaneous amplitude profiles. Gas hydrates and a prominent BSR were discovered in the region of gas chimney occurrence. Logging data and sample analyses from drill holes over the chimney structures indicate close relationships between active fluid flow and the formation and accumulation of gas hydrates. Our results document that gas chimneys consisting of a connected network of fractures provide a passageway along which fluids ascend beneath the gas hydrate stability zone (GHSZ) for hydrate formation in fine-grained sediments of the northern South China Sea. Fluids accumulate in uppermost part of the slope where they are trapped beneath the prograding submarine delta sequence, which is not permeable enough for fluids to migrate through. Gas-charged fluids may originate from deep-seated hydrocarbon reservoirs, which are indicated by the molecular and isotopic signatures of gases in gas hydrates occurrence zone. We suppose that the fractures extend into the GHSZ, creating a space for hydrate condensation in the fine-grained sediments of the slope. Gas chimneys may account for the overpressured fluid activity and, most likely, the gas hydrate enrichment in the northern SCS. © 2012 Elsevier B.V. Source

Zhang Y.,China University of Geosciences | Zhang Y.,Ocean University of China | Su X.,China University of Geosciences | Su X.,Ocean University of China | And 7 more authors.
Frontiers of Earth Science | Year: 2012

Candidate division JS1-and Chloroflexi-related bacteria are ubiquitous in various deep marine sediments worldwide, yet almost nothing is known about their abundance and diversity in cold seep sediments. Here, we investigated the abundance and diversity of JS1- and Chloroflexi-related bacteria in a cold seep marine sediment core collected from the northern South China Sea (SCS) with the employment of quantitative polymerase chain reaction (qPCR) and 16S rRNA gene phylogenetic analyses. The qPCR results showed that 16S rRNA gene copies per gram of sediments for the total bacteria and JS1- and Chloroflexi-related bacteria were at magnitudes of 10 8 and 10 6, respectively. The relative abundance of JS1- and Chloroflexi-related 16S rRNA genes to that of total bacteria was 0. 07%-8. 78% throughout the core. Phylogenetic analyses showed that the JS-1 related clone sequences were dominant throughout the core. Our study provided insights into abundance and diversity of JS1- and Chloroflexi-related bacteria in the northern SCS cold seep sediments. © 2012 Higher Education Press and Springer-Verlag Berlin Heidelberg. Source

Chen D.,CAS Qingdao Institute of Oceanology | Chen D.,University of Chinese Academy of Sciences | Wu S.,CAS Qingdao Institute of Oceanology | Dong D.,CAS Qingdao Institute of Oceanology | And 3 more authors.
Chinese Journal of Oceanology and Limnology | Year: 2013

The origin and migration of natural gas and the accumulation of gas hydrates within the Pearl River Mouth Basin of the northern South China Sea are poorly understood. Based on high-resolution 2D/3D seismic data, three environments of focused fluid flow: gas chimneys, mud diapirs and active faults have been identified. Widespread gas chimneys that act as important conduits for fluid flow are located below bottom simulating reflections and above basal uplifts. The occurrence and evolution of gas chimneys can be divided into a violent eruptive stage and a quiet seepage stage. For most gas chimneys, the strong eruptions are deduced to have happened during the Dongsha Movement in the latest Miocene, which are observed below Pliocene strata and few active faults develop above the top of the Miocene. The formation pressures of the Baiyun Sag currently are considered to be normal, based on these terms: 1) Borehole pressure tests with pressure coefficients of 1. 043-1. 047; 2) The distribution of gas chimneys is limited to strata older than the Pliocene; 3) Disseminated methane hydrates, rather than fractured hydrates, are found in the hydrate samples; 4) The gas hydrate is mainly charged with biogenic gas rather than thermogenic gas based on the chemical tests from gas hydrates cores. However, periods of quiet focused fluid flow also enable the establishment of good conduits for the migration of abundant biogenic gas and lesser volumes of thermogenic gas. A geological model governing fluid flow has been proposed to interpret the release of overpressure, the migration of fluids and the formation of gas hydrates, in an integrated manner. This model suggests that gas chimneys positioned above basal uplifts were caused by the Dongsha Movement at about 5. 5 Ma. Biogenic gas occupies the strata above the base of the middle Miocene and migrates slowly into the gas chimney columns. Some of the biogenic gas and small volumes of thermogenic gas eventually contribute to the formation of the gas hydrates. © 2013 Chinese Society for Oceanology and Limnology, Science Press and Springer-Verlag Berlin Heidelberg. Source

Tang C.,CAS South China Sea Institute of Oceanology | Tang C.,CAS Yantai Institute of Coastal Zone Research | Zhou D.,CAS South China Sea Institute of Oceanology | Endler R.,Leibniz Institute for Baltic Sea Research | And 2 more authors.
Journal of Marine Systems | Year: 2010

The Pearl River Estuary in the Southern China was studied both by applying concepts of seismic stratigraphy to the interpretation of high resolution seismic profiles and by correlating with borehole records. The correlation between seismic facies and borehole stratigraphy of the estuary enables to propose a seismic stratigraphy model of the estuarine infill. The stratigraphy and evolution of the Holocene succession of the estuary were reconstructed. The history of estuarine sedimentary development consists of five stages represented by 5 seismic units bounded by laterally sub-continuous seismic interfaces and being consistent with the 5 borehole subdivisions: (i) The basal stage deposits, represented by BU and the borehole section P, might represent the paleo Pearl River alluvial deposits in the Late Quaternary. (ii) The stage I deposits, represented by SU1 and the borehole section A, might be late glacial prodeltaic deposits that occurred during Marine Isotope Stage 3 highstand. (iii) The stage II deposits, represented by SU2 and the borehole section B, consist of relatively coarse-grained sediments deposited during the post glacial transgression about 20-10. ka BP. (iv) The stage III deposits (SU3 and the borehole section C) were generated when the rate of sea level rise decreased in ~. 20-10. kz BP, which forced sediments to be deposited inside the estuary, where a tidal ravinement surface was characterized by strong erosions and channel formations in the outer zone of the estuary. (v) The stage IV deposits (SU4 and the borehole section D) are the infillings of the estuarine highstand progradation during the last 6000. yrs when the sea surface almost reached the present level. © 2010 Elsevier B.V. Source

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